A single-crystal silicon wafer as a semiconductor device substrate is cut out from a single-crystal silicon ingot, and is produced by being subjected to various physical, chemical, and thermal treatments. The single-crystal silicon ingot is usually obtained by the Czochralski method (hereinafter referred to as “CZ method”) in which a seed crystal is dipped into silicon melted in a quartz crucible and pulled up to grow a single crystal. However, micro defects called Grown-in defects are induced in the crystal during the single crystal growth.
The Grown-in defects depend on a pulling-up speed during the single crystal growth and a temperature distribution (temperature gradient in crystal in a pulling-up axis-wise direction) in the single crystal immediately after solidification. In the single crystal, the Grown-in defects appear in the form of a hole aggregation defect called COP (Crystal Originated Particles), having sizes ranging from about 0.1 to 0.2 μm, or in the form of a defect including a micro dislocation called a dislocation cluster, being about 10 μm in size.
In the single-crystal silicon wafer produced by the CZ method, an Oxidation-induced Stacking Fault (hereinafter referred to as “OSF”) appearing in a ring shape may be generated when the single-crystal silicon wafer is subjected to a high-temperature oxidation heat treatment. A potential area where the OSF ring is generated depends on a thermal history of the crystal during growth, particularly influenced by a pulling-up speed during growth. The region where the OSF ring appears gets shrunk from the outer circumferential side to the inner side of the crystal as the pulling-up speed is lowered.
In other words, the inner side area of the OSF ring spreads to the whole area of wafer when the single crystal is grown at a higher speed, and the outer side area of the OSF ring spreads to the whole area of wafer when the single crystal is grown at a lower speed.
In the case where OSF exists on a wafer surface which is of a device activation area, the OSF causes a leak current to deteriorate a device characteristic. COP is a factor which lowers an initial oxide-film withstand voltage, and the dislocation cluster also causes a defective characteristic of the device formed therein.
Therefore, the single crystal is conventionally grown at a high pulling-up speed such that the ring-shaped OSF generation region is located in the outer circumferential portion of the crystal. For example, as described in Japanese Patent Application Publication No. 2002-145698, there is proposed a wafer in which the OSF region is widely distributed from a circumferential edge portion to a central portion of the wafer and a micro COP region is formed inside the OSF region.
However, a single-crystal silicon wafer (hereinafter referred to as “defect-free crystal silicon wafer”) in which the number of Grown-in defects including extremely small COPs is decreased as much as possible is produced with the advance of the fine process of the semiconductor device to cope with growing demand on miniaturization and high performance.
Accordingly, a COP evaluation is made in the defect-free crystal silicon wafer to make an acceptance determination in which crystal integrity (defect-free) is guaranteed by the number of defects (COPs) and the presence or absence of a specific pattern through the COP evaluation. In the COP evaluation, a method called a copper deposition method (copper decoration method) can be employed as an example of the method for detecting COPs.
In the copper deposition method, an uneven insulating film (oxide film) is utilized in a region where the defects (COPs) exist when the oxide film is formed on a wafer surface. After the oxide film having a predetermined thickness is formed on the wafer surface, an external voltage is applied, copper is deposited while the oxide film is destroyed in a defect region on the wafer surface, and the deposited copper is observed to detect the defects (COPs) by the naked eye or with a Transmission Electron Microscope (TEM) or Scanning Electron Microscope (SEM).
The COP generation factors can be classified into the crystal-induced and the non-crystal-induced. The crystal-induced COPs mean the Grown-in defects that are induced into the crystal during the single crystal growth.
According to the investigation thus far, it is found that the generation patterns of the crystal-induced COPs are classified into the following four segments:
(1) The crystal-induced COPs appear in a disc shape in a central part of wafer.
(2) The crystal-induced COPs appear in a ring shape in an peripheral part of wafer as following its circumference.
(3) The patterns (1) and (2) simultaneously appear, namely, appearing in a disc-ring shape.
(4) The crystal-induced COPs densely appear in the whole surface of the wafer (300 counts or more in a wafer of 300 mm in diameter).
On the other hand, the non-crystal-induced COP is not COP in a strict sense, and caused by a micro flaw or scratch generated on the wafer surface during handling the silicon wafer. When the non-crystal-induced COPs are observed with a surface defect inspection apparatus (for example, SP2: product of KLA-Tencor) or by the copper deposition method (copper decoration method), the COPs are generated in a line shape, or in a dot shape locally or in the whole surface of the wafer.
Since the non-crystal-induced COP is not an intrinsic defect derived from the silicon single crystal itself, which is a constituent of a wafer, the non-crystal-induced COPs should be removed from the COP evaluation object. In the current COP evaluation, COPs which are easily determined as the non-crystal-induced are removed from the evaluation.
However, there is no proper method for distinguishing the non-crystal-induced COP and the crystal-induced COP from each other (that is, both are identified to thereby allow the non-crystal-induced COPs to be excluded in the evaluation), in particular, there is no method for determining that the non-crystal-induced COPs appearing in a dot shape in the whole surface of the wafer are irrelevant to crystal-induced COPs.